The present invention relates to an auto-drain unit for compressed gas supply systems, in particular pneumatic supply systems.
In pneumatic supply systems, for example, the compressed air generated by the system generator includes, inter alia, entrained water. This water has to be removed before reaching the points of use.
It is common to use filter assemblies to extract the water from compressed air delivered through pneumatic supply systems, with the filter assemblies typically being mounted in modules which often include additional devices, such as pressure regulators and oil-mist lubricators. These filter assemblies include a hydrophobic filter element on which water agglomerates and a bowl in which the water is collected. In some designs, the filter assemblies are configured to cause the compressed air stream to form a vortex in the filter bowl, which vortex creates a centrifuging action and separates water from the compressed air stream.
In such filter assemblies, an auto-drain unit is provided for automatically discharging collected water from the filter bowl before the water level can cause flooding of the filter element. Several auto-drain units are available which provide for the automatic discharging of collected water when the water level reaches a predetermined level, as disclosed for example in U.S. Pat. Nos. 5,636,655 and 5,595,210.
The existing auto-drain units typically incorporate a main discharge valve which is actuatable to discharge collected water from the filter bowl, a trigger valve for actuating the discharge valve, and a light polymer float for actuating the trigger valve. The float is configured to be freely movable vertically in the filter bowl such that, as the water level rises in the filter bowl, the flotation force acting on the float causes the float to rise until such point that the trigger valve is actuated when the collected water reaches a predetermined depth. When the trigger valve is actuated, the system pressure acts on the discharge valve to actuate the same and cause the collected water together with some compressed air to be discharged from the filter bowl.
The use of a float to actuate the trigger valve has, however, a number of drawbacks, as the flotation force, which is the weight of water displaced by the float less the weight of the float itself, is determined by the size of the float, which governs the water displacement, and the weight of the float. The physical size of the float is constrained as the float must be able to fit into the smallest filter bowls commercially available in order to allow for general application. Also, since the float has some non-negligible weight, even when formed of a light polymeric material, the force achievable from a given volume of water is reduced by that weight. Thus, the floatation force which can be developed, given the possible float materials and size, is relatively small, typically less than 10 g. In existing designs, this small force has to be mechanically amplified by the use of a lever arrangement to ensure reliable operation of the trigger valve.
In addition, these existing auto-drain units are also prone to damage because of the delicate lever arrangement and the light float, not least when exposed to the slug of water swept into the filter bowl during system start-up.
Furthermore, the construction of the float is constrained by the fact that the system pressure can continually vary over a wide range of pressures, and particularly at system shut-down. For example, a system working nominally at 10 barg can fall to 0 barg when shut down. If the float is hollow, the float will experience large differential pressure forces across the walls of the float. These pressures would cause the float to split and render the auto-drain unit useless, and thus the float has to be specially configured, which introduces additional complexity and cost. This is a common design weakness in commercial auto-drain units. Other float constructions are used, for example, floats made from a closed-cell polymer foam, which are sufficiently strong to withstand the changing pressure forces without damage and do not take up water or oil. However, this material is expensive.
Another problem with the existing auto-drain units arises from the fact that the water collected from pneumatic supply systems is rarely clean. Collected water usually contains oil and particles of dirt and/or rust, and can also contain algae and other organic growths. When exposed to such contaminants, small orifices can become blocked, sliding parts can suffer from stiction problems and the component materials can be chemically degraded.
A further drawback of the existing auto-drain units is that the height of the column of water required to actuate the trigger valve is variable and dependent on the system pressure. In the existing auto-drain units, for a circular trigger valve orifice, the closing force F is given by F=8PD2, where P is the pressure differential across the trigger valve and D is the orifice diameter. Thus, the closing force F acting on a trigger valve orifice of diameter 0.5 mm at a pressure differential of 12 bar is 24 gf. In this case, a 4:1 or 5:1 mechanical advantage would be required to ensure that the float would operate the trigger valve reliably over the possible system pressure range. This dependence can be reduced by providing the trigger valve orifice as a very small orifice, for example, less than 0.5 mm, thereby reducing the relative effect of the pressure differential. This, however, creates manufacturing and design problems, as such small orifices having a defined gas conductance are very difficult to manufacture. There are also reliability issues as small orifices are more easily blocked with oil or debris. Also, the surface tension of the water is a significant factor where small orifices are utilized.
It is an aim of the present invention to provide an improved auto-drain unit for use in compressed gas supply systems, in particular pneumatic supply systems. In a preferred aspect, it is an aim of the present invention to provide an auto-drain unit which is pressure balanced and is triggered by the same volume of collected liquid irrespective of the system pressure.
Accordingly, the present invention provides an auto-drain unit for a compressed gas supply system, comprising: a first, main chamber including a liquid collection reservoir; a second, reference chamber; a discharge valve actuatable to discharge collected liquid from the liquid collection reservoir; a trigger mechanism for actuating the discharge valve; a diaphragm at least in part defining the liquid collection reservoir and the reference chamber, the diaphragm being configured to be movable under the weight of liquid collected in the liquid collection reservoir and operate the trigger mechanism when the weight of collected liquid exceeds a predetermined threshold; and a fluid conduit fluidly connecting the reference chamber to a location in the main chamber above the maximum possible liquid level therein.
Preferably, the diaphragm is slack and able to move freely when loaded.
More preferably, the diaphragm is shaped such the chord length is greater than the lateral dimension.
Preferably, the diaphragm has a thickness of not more than about 50 xcexcm.
More preferably, the diaphragm has a thickness of not more than about 30 xcexcm.
Preferably, the trigger mechanism comprises a trigger valve for actuating the discharge valve, the trigger valve being actuated when the weight of collected liquid exceeds a predeterminable threshold.
More preferably, the trigger valve includes a paddle unit which includes a paddle member disposed adjacent the diaphragm such as to be acted upon by the diaphragm when liquid collects in the liquid collection reservoir, the paddle unit being movable between a first, non-actuated position and a second, actuated position.
Preferably, the surface of the paddle member adjacent the diaphragm is a convex surface.
More preferably, the surface of the paddle member adjacent the diaphragm is a part spherical surface.
Preferably, the trigger valve includes a biasing element for biasing the paddle unit to the first position.
In one embodiment the paddle unit is pivotally supported such as to be pivoted when the weight of liquid acting on the paddle member exceeds a predetermined threshold.
In one preferred embodiment the paddle unit supports a valve pad for opening or closing a valve seat.
In another preferred embodiment the trigger valve further includes a sealing valve unit coupled to the paddle unit, the sealing valve unit including a piston and a valve pad mounted thereto for opening or closing a valve seat.
In another embodiment the paddle unit is axially movable such as to be moved axially when the weight of liquid acting on the paddle member exceeds a predetermined threshold.
In a preferred embodiment the paddle unit includes a bobbin which is slideably disposed in a valve sleeve, the bobbin being movable in the valve sleeve between actuated and non-actuated positions.
Preferably, the trigger valve is pressure balanced.
Preferably, the fluid conduit comprises an annular conduit.
More preferably, the auto-drain unit further comprises an annular element defining a cavity and including the fluid conduit at the periphery thereof.
Preferably, the annular element comprises first and second co-axial tubular sections of different lateral dimension, with the fluid conduit being defined therebetween.
Preferably, the diaphragm extends laterally across the cavity.
Preferably, the compressed gas supply system is a pneumatic supply system.
The present invention also extends to a filter assembly incorporating the above-described auto-drain unit.
With this arrangement, the present invention advantageously provides an auto-drain unit which is triggered by the weight of a collected column of liquid, and does not incorporate a float or require the trigger valve to be submerged in the collected liquid. In a preferred embodiment the trigger valve is pressure balanced such that the weight of liquid required to actuate the auto-drain unit does not vary in dependence on the system pressure, thereby enabling the use of valve orifice sizes of greater dimension.
Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: